1. Field of the Invention
The present invention relates to a secondary battery, and more particularly, to a rectangular secondary battery having a cap assembly with improved terminal members.
2. Description of the Related Art
Recently, chargeable secondary batteries adoptable for miniaturized cellular phones, notebook computers, compact camcorders and the like have been studied. In particular, lithium ion batteries have rapidly been introduced in the market in the light of their high energy density per unit weight.
In such lithium ion batteries, lithium cobaltate (LiCoO2) is mainly used as a positive active material, and a carbon material is used as a negative active material. Recently, some batteries using lithium manganese oxides having a spinel structure as a positive active material have also been developed. Secondary batteries are classified into liquid electrolyte batteries and polymer electrolyte batteries according to the kind of electrolyte used. Batteries using a liquid electrolyte are generally referred to as lithium-ion batteries, and batteries using a polymer electrolyte are referred to as lithium-polymer batteries. The lithium-ion batteries are manufactured in various shapes, typically cylindrical and rectangular shapes.
FIG. 1 shows an example of a rectangular secondary battery 10 commercially available in the market.
Referring to the drawing, the rectangular secondary battery 10 has a case 11, and an electrode assembly 12 inside the case 11. The electrode assembly 12 is formed such that a positive electrode plate and a negative electrode plate are alternately stacked and a separator is interposed therebetween as an insulator. The case 11 is hermetically sealed by a cap assembly 20. The cap assembly 20 is welded to the case 11 to hermetically seal the case 11. The cap assembly 20 includes a cap plate 21, welded to the case 11 to hermetically seal the case 11, having a throughhole 21a, a gasket 24 and an insulating plate 25, respectively positioned in the lower and upper portions of the cap plate 21, and riveted by a terminal member 23 penetrating the same, and a terminal plate 26, installed on the bottom surface of the insulating plate 25, supported by the terminal member 23 and connected with a positive electrode tab 12a of the electrode assembly 12.
In the aforementioned rectangular battery, the throughhole 21a in the cap plate 21 is hermetically sealed by the gasket 24 and the insulating plate 25 is riveted by the terminal member 23 to be combined with the cap plate 21. However, since the bonding strength of the terminal member 23 is weak, the sealing capability is poor. In order to ensure complete sealing of the throughhole 21a by the gasket 24 and the insulating plate 25, 30 to 40% compression of the gasket 24 should be maintained by the bonding strength between the gasket 24, the cap plate 21, the insulating plate 25 and the terminal plate 26 by the terminal member 23. In order to attain light-weight secondary batteries, the terminal member 23 may be aluminum. Also, in order to improve the sealing capability, a head 23a of the terminal member 23 may be extended in a longer side direction. However, in these cases, the bonding strength of the terminal member 23 is reduced, thereby lowering the sealing capability of the throughhole 21a.
If the sealing capability of a throughhole, in a cap plate, through which a terminal member is drawn out, is lowered, leakage of the electrolyte occurs, which shortens the lifetime of the battery and causes contamination.
Another example of a cap assembly for a rectangular secondary battery is disclosed in U.S. Pat. No. 5,985,478 and Japanese Patent Laid-Open Publication Hei 11-329407.
In the disclosed structure, a throughhole in a negative electrode plate is simply riveted using a rivet connected with a tab in a state in which an insulator and a gasket are positioned in the upper and lower portions of the negative electrode plate. This structure increases the sealing capability of the throughhole but there is a limitation.